Generation of microwave frequency combs with narrow line spacing

ABSTRACT

A harmonic generator such as a step recovery diode device is driven by two or more input signals of different frequencies to produce a comb spectrum consisting of lines at multiples of the input signal frequencies interspersed with lines at the frequencies of the intermodulation products.

o I I Umted States Patent 11 1 [111 3,777,271 Telewski Dec. 4, 1973GENERATION 0F MICROWAVE 3,401,355 9/1968 Kafitz 331 53 FREQUENCY COMBSWITH NARROW FOREIGN PATENTS OR APPLICATIONS LINE SPACING 645,706 11/1950Great Britain 331/37 [75] Inventor: Frederick John Telewski, Portland,99, 11/1957 Canada Oreg. 642,253 11/1960 Italy 331/37 [73] Assignee:Cutler-Hammer, Inc., Milwaukee, OTHER PUBLICATIONS 1 M011 PhysicalModeling of the Step Recovery D1- [22] Filed: Oct 4 1971 ode Proceedingof IEEE Vol. 57 No. 7 July 1969 Pages 1250-1259 Appl' 186367 BerryVariable-Width Fractional-Nanosecond Pulse Generators 1962 InternationalSolid State Circuits 52 us. (:1 328/16, 307/281, 331/76 Conference Pages80-81 [51] Int. Cl. 1101b 19/00 Gottlieb Harmonic Generation With theStep Recov- [58] Field of Search .....328/14-16; ery Diode BookFrequency Changers 1st Edition Jan.

3 3 1 7, 53, 7 3- 9 65 Pages 87-96 307/281; 332/17, 16, 22 KrakauerHarmonic Generation, Rectification Proceeding of lEEE July 1962 Pages1665-1676 [56] References Cited UNITED STATES PATENTS PrimaryExaminer-John W. Huckert 2,028,212 1 1936 Heising 331 76 Assistant3,448,401 6/1969 We1ch 331 53 oy Huff 2,500,945 3/1950 Hansen 332/172,459,822 1/1949 Lalande 331/53 57 ABSTRACT 3,059,187 10/1962 Jaffe328/16 3,054969 9/1962 Harrison 331/76 A harmonic generator such as astep recovery d1ode 3,164,802 1/1965 Kl i 331 /37 device is driven bytwo or more input signals of differ- 3,379,995 4/1968 Koontz..... 331/531 ent frequencies to produce a comb spectrum consist- 3,576,499 4/1971 f33 /7 ing of lines at multiples of the input signal frequencies 2-4023856/1946 Eaton 331/53 interspersed with lines at the frequencies of theinter- 3,199,047 8/1965 Cushman 331/76 modulation products 3,296,5491/1967 Johnson 331/53 3,312,909 4/1967 Bryant .v 331 /76 6 Claims, 4Drawing Figures 7 8 7 4 7 OSCILLATOR IMPULSE g g fl f' HARMONIC FsGENERATOR "'GENERATOR PATENTEDBEE 4% 3,777,271 A I 2 OSCILLATOR HARMONICCOMBINER 27 GENERATOR A OSCILLATOR v F2 FREQUENCY HARMONIC MULTIPLI ERMODULATOR GENERATOR OSCILLATOR 5 a 7 ,4 7 OSCILLATOR IMPULSE BANDPASSHARMONIC Fs GENERATOR fig-Q? GENERATOR I0 I3 1/ I2 o I v D A j aFREQUENCY, GHz

. l GENERATION OF MICROWAVE FREQUENCY COMBS WITHNARROW LINE SPACING,

BACKGROUND are often called combs, and devices for producing 1 them arecalled comb generators. Y

2. Prior Art A well-known type of microwave comb generator comprises astep recovery diode (SRD)v and associated circuit elements arranged obedriven'by a. repetitive input signal such as a I ve and "produce a trainof very short pulses repetitive at the input signal frequency. Theresulting comb contains signal components, called lines, atfrequencieswhich'are consecutive integral multiples of the input signalfrequency.

SRD multipliers operate well when driven. at freqiie cies above aboutMHz. At lower frequencies the cf ficiency becomes too low for usefulcomb generation, owing to recombination of minoritycarriers near thediode junction. Accordingly the minimum combline spacing that can beobtained with reasonable efficiency using prior art SRD generators'is.about .10 MHz.

Another limitation on. the'closeness of line spacing in. prior art combgeneratorsresults fromthe fact that the amplitude generally decreaseswith increasing harmonic order, becoming too low to be useful in linesabove the 150th to 200th, for example. Therefore the minimum obtainablecomb line spacing in any event is about one half to one percent of thefrequency of the uppermost useful comb line.

SUMMARY According to this invention, the foregoing obstacles toefficient generation of close-spaced microwave combs are avoided byoperating the harmonic generator also as an intermodulation generator. Aconven-- tional comb generator, preferably of the SRD type, is drivensimultaneously by two or more signals of different frequencies toproduce two or more different combs with line spacings corresponding tothe respective drive signal frequencies. Thecomb generator is inherentlya non-linear device and therefore produces the intermodulation productsof the different combs, resulting in a composite comb consisting oflines that are spaced by the difference between the frequencies of thedriving signals. Since the line spacing of the composite comb depends onthe difference between the drive signal frequencies, these frequenciesmay be chosen within a range-cf efficient comb generator operation.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a block diagram of a simpleembodiment, illustrating the basic principle, of the invention.

FIG. 2 is a block diagram of a modification of the em- DESCRIPTION OFTHE PREFERRED EMBODIMENTS Referring to FIG. 1, two oscillators l and 2,operating at respective different radio frequencies F1 and F2, areconnected'to a combiner 3. The resultant output of the combinerisapplied to a harmonic generator 4. The harmonic generator 4 may be aprior art comb generator, for example one of the SRD type described at 0pages 89-92 in the Hewlett-Packard catalog of Solid ear passive device,such as a bridge network or a hybrid,for merelycoupling both oscillators1 and 2 to the single input portof the harmonic generator 4. Theoscillators. 1 and 2 are designed to operate at frequencies of, say,MI-Iz and 102 MHz respectively, and the harmonic generator 4 is designedto operate with an input frequency of about 100 MHz, like the Hewlett-Packard module 32002A described in the above mentioned catalog.

In the operation of the embodiment of FIG. 1, the harmonic generator 4operates in normal manner in response to oscillator 1 to produce a combconsisting of lines at; 100: MHZ intervals, and in response tooscillator 2 toproduceanother comb of lines at 102 MHz intervals. Asmall'portion of the composite comb, in the region from 3.0 to,3.1GI-Iz, is shown in FIG. 4. The heavy lines 10 and 12 represent the30th'and 31st harmonics respectively of 100MHz, and the heavy line 11represents the 30th'harmonic of 102 MHz, at 3.06 GHz. Such lines arereferred to hereinafter as primary comb lines.

T ee l lines 10, 11 and 12 would be the only ones pre'se ,m theillustrated position of the composite comb if the component 100 MHz and102 MHz combs were generated separately and passively combined. Becauseboth combs are generated in the same non-linear device, intermodulationoccurs between pairs of elementary components-of different frequenciesthat are present or generated in the device, producing additionalcomponents whose frequencies are the sums and differences of thefrequencies of the respective pairs. The additional components appear atsuccessive frequencies that differ by the smallest difference betweenthose of original generating components, in this case 2 MHz. The finerlines in FIG. 4, collectively designated by the reference numeral 13,represent the 48 additional secondary comb lines between lines 10 and 12that are produced by intermodulation. I

The signal in eachcomb line 13 is the summation of a large number ofdifferent intermodulation products, all of the same frequency. Forexample, the line at 3.062 GI-lz includes the sum frequency product atthat frequency formed by line 11 at 3.06 GHz and a signal component at 2MHz which in turn is the difference frequency product of 100 MHZ and 102MHz. The 3.062 GHz line also includes the difference frequency productof line 12 at 3.1 GHz and a component at 38 MHz, which in turn is the19th harmonic of the aforementioned 2 MHz component. Anotherintermodulation product at 3.062 GHz results from the difference between31st harmonic of 102 MHz, at 3162 MHz, and the 100 MHz fundamental.These and many other such intermodulation products, all at 3.062 GHz,combine to produce the resultant comb line signal at that frequency.Secondary comb line signals are formed in similar manner at 2 MHzintervals throughout the entire comb line spectrum.

The amplitude of any particular intermodulation product depends upon,and is lower than the amplitudes of the signal components from which itis produced. Successively higher order harmonic intermodulation productsare of successively lower amplitude. Each composite comb line signal hasa respective amplitude that depends upon the amplitudes of itsconstituents, and upon how they are combined to produce it. In general,the nearer a secondary comb line is to one of the primary lines, thegreater its amplitude. Typically, the weakest secondary comb line signalmay be of the order of db below the nearest primary comb line signal.

Unless the oscillators l and 2 are phase locked, or controlled by acommon frequency determining means, the phase relationship between theintermodulation products of a given frequency will vary more or less atrandom, producing corresponding time variations in amplitude of theindividual secondary comb lines. Such variations may be desirable insome applications, but generally it is preferable to maintain constantamplitudes. To this end, the oscillators may be phase locked in any ofseveral well known manners, orthedesired drive signals may be obtainedfrom a single original source.

Referring to FIG. 2, an oscillator 5 is designed to operate at afrequency Fs equivalent to the desired line spacing, say 2 MHz. Theoutput of oscillator 5 is supplied to a frequency multiplier 6 and alsoto an amplitude modulator 7. The frequency multiplier 6 is designed inknown manner to provide an output of frequency NFs, where N is, forexample, 50.

The 100 MHz output of multiplier 6 is amplitude modulated in themodulator 7 by the 2 MHz input from oscillator 5, to produce an outputconsisting of a 100 MHz carrier and the lower and upper sidebands at 98MHz and 102 MHz. This output is applied to the harmonic generator 4,which operates essentially as in the system of FIG. 1 to produce acomposite comb with 2 MHz line spacing. The phase relationships betweenthe intermodulation components remain substantially constant because thedrive signals are obtained from the same ultimate source, the oscillator5.

Referring to FIG. 3, the above described frequency multiplier andamplitude modulator are replaced by an impulse generator 8 which isdriven by the oscillator 5 to produce a train of brief pulses recurrentat the frequency Fs, resulting in a frequency comb containing lines ofappreciable amplitude in the region of NFs, say from 96 to 104 MHz.

The impulse generator 8 may be similar to the harmonic generator 4, butdesigned to operate at lower input and output frequencies. ItsCffiClCl'lC); may be quite low compared to that of the generator 4, butmay be improved to some extent by means of a resonator incorporated inthe device and operating in known 1. The method of generating amicrowave frequency comb with narrow line spacing, comprising the stepsof: a. producing signals that differ in frequency by the desired linespacing, b. generating harmonics of said signals, and c. generatingintermodulation products of said harmonics. 2. The method of generatinga microwave frequency "comb with narrow line spacing, comprising thesteps of:

a. producing a signal of a frequency equal to the desired line spacing,

b. utilizing said signal to produce at least two further signals thatdiffer in frequency by the desired line spacing,

c. generating harmonics of said further signals, and

d. generating intermodulation products of said harmonies.

3. The method of operating a step recovery diode comb generator whichcomprises driving said generator at two frequencies that differ fromeachother by the desired comb line spacing. I

4. The method set forth in claim 3, wherein said two frequencies areintegral multiples of said comb line spacing.

5. Apparatus for generating a microwave frequency comb with narrow linespacing Fs, comprising:

a. a harmonic generator adapted to be driven by signals of frequenciesin the vicinity of NFs, where N is a number substantially greater thanunity,

b. means for producing at least two drive signals of frequencies in thevicinity of NFs that differ in frequency by the desired line spacing Fs,and

0. means for applying said drive signals simultaneously to said harmonicgenerator.

6. The invention set forth in claim 5, wherein said harmonic generatoris a step recovery diode device, and said means for producing said drivesignals comprises an oscillator of frequency Fs;-an impulse generatordriven by said oscillator, and bandpass filter means designed to passfrequencies in the vicinity of NFs.

* I! I t I

1. The method of generating a microwave frequency comb with narrow linespacing, comprising the steps of: a. producing signals that differ infrequency by the desired line spacing, b. Generating harmonics of saidsignals, and c. generating intermodulation products of said harmonics.2. The method of generating a microwave frequency comb with narrow linespacing, comprising the steps of: a. producing a signal of a frequencyequal to the desired line spacing, b. utilizing said signal to produceat least two further signals that differ in frequency by the desiredline spacing, c. generating harmonics of said further signals, and d.generating intermodulation products of said harmonics.
 3. The method ofoperating a step recovery diode comb generator which comprises drivingsaid generator at two frequencies that differ from each other by thedesired comb line spacing.
 4. The method set forth in claim 3, whereinsaid two frequencies are integral multiples of said comb line spacing.5. Apparatus for generating a microwave frequency comb with narrow linespacing Fs, comprising: a. a harmonic generator adapted to be driven bysignals of frequencies in the vicinity of NFs, where N is a numbersubstantially greater than unity, b. means for producing at least twodrive signals of frequencies in the vicinity of NFs that differ infrequency by the desired line spacing Fs, and c. means for applying saiddrive signals simultaneously to said harmonic generator.
 6. Theinvention set forth in claim 5, wherein said harmonic generator is astep recovery diode device, and said means for producing said drivesignals comprises an oscillator of frequency Fs, an impulse generatordriven by said oscillator, and band pass filter means designed to passfrequencies in the vicinity of NFs.